Conventional alphanumeric cathode ray tube display terminals, such as the IBM 3277, 3278 and 3279 display stations, use a display refresh buffer storing coded representations of the characters or symbols to be displayed on a raster-scanned cathode ray tube display. The display is refreshed by periodically reading the codes from the display refresh buffer and using these codes to access a character generator which includes a store which contains the actual bit patterns needed to display the characters or symbols. The character generator store need only store the bit patterns for a particular character or symbol once, no matter how many times that character is to be displayed. If the character generator store is a writable one such as is the case of the IBM 3279 and 8775 display stations, (see for example U. S. Pat. No. 4,245,308 and 4,278,973) graphics pictures can be displayed using the so-called character graphics technique. In this technique, the graphics picture is built up from a number of special characters or symbols. Codes representing these characters are stored in the display buffer and the corresponding bit patterns are stored in the writable memory of the character generator. Once the appropriate codes and bit patterns have been loaded into the display buffer and character generator once, operation is like the conventional alphanumeric display. It will be seen, therefore, that the codes stored in the display buffer are pointers to the required bit patterns.
Another type of graphics display apparatus (see, for example, co-pending commonly-assigned US Patent Application Ser. No. 748,259) uses the so-called bit buffer approach. In this arrangement, each picture element (pel or pixel) on the display screen is associated with a minimum storage requiremet of at least one bit for monochrome or at least 3 bits for color. Thus a display capable of displaying 1000.times.1000 pels would need at least a 1M bit buffer if monochrome or at least a 3M bit buffer if color. Typical of such bit-for-pel buffered graphics displays are the IBM 5080, 3270PC-GX and 3270 PC-AT/GX displays.
Some modern displays, such as the IBM 3270 PC-G and 3270 PC-AT/G displays, use a bit-for-pel buffer for displaying graphics and a coded display buffer and character generator for displaying alphanumeric characters: in this case the character generator store used need not to be a writable store since it will only be used to display "standard" characters or symbols. Co-pending commonly-assigned US patent application Ser. No. 708,755, now U.S. Pat. No. 4,686,521 describes how the graphics and alphanumeric data can be mixed.
The programmed symbol or character graphic technique of displaying graphic images, despite being very efficient in its use of random access memory (RAM), suffers from three disadvantages. Firstly, the RAM must be able to cycle at character display rates, typically 270 nsec for a high quality color display. Secondly, a graphics processing routine can take as much as 60% of its time pre-allocating the programmed symbol cells in the display buffer. Thirdly, the complexity of detail that may be displayed is limited by the number of programmed symbols available: the display processor can run out of spaces in the character generator memory. U.S. Pat. No. 4,308,532 summarises the advantages and disadvantages of various graphic display techniques and proposes a solution to the problems associated with a character graphics system when the character generator memory is full. US Patent 4,308,532 proposes that when the character generator memory is full, the graphics cells be displayed at lower resolution to create free space in the character generator. In order to avoid certain bandwidth restrictions, it also proposes splitting the character generator into two sections, one called the even-cell generator and the other the odd-cell generator. The odd-cell generator contains bit patterns corresponding the odd-numbered columns of pels and the even-cell generator contains bit patterns corresponding to even-numbered columns of pels. Normally the output of the odd-cell and even-cell generators would be interleaved but to display at low resolution only the odd or even-cell output is used.